Omnidirectional antenna

ABSTRACT

A substantially hemispherical pattern of circularly polarized radiation is obtained by quadrature-phase feeding of specially shaped dipole-and-reflector assemblies in orthogonal vertical planes, each having a generally hemispherical amplitude pattern of linearly polarized radiation. The hemispherical pattern of each assembly is obtained by arcuate downwardly bent dipole elements each of physical length greater than a quarter wavelength and a horizontal reflector rod of length exceeding a half wavelength, spaced downwardly of the dipole feed-point by substantially more than a quarter wavelength, but spaced downwardly of the tip portions of the dipole elements by substantially less than a quarter wavelength.

Elted States Patent 1 Dienes [451 Nov. 6, 1973 OMNIDIRECTIONAL ANTENNA [75] Inventor: Geza Dienes, Claremont, Calif.

[73] Assignee: Andrew California Corporation,

Claremont, Calif.

[22] Filed: May 14, 1971 [21] Appl. No.: 143,525

3,348,228 10/1967 Melancon 343/908 3,388,400 6/1968 Veldhuis 3,514,780 5/1970 Petrick et al. 343/741 Primary ExaminerEli Lieberman Attorney-C. Frederick Leydig et al.

[57] ABSTRACT A substantially hemispherical pattern of circularly polarized radiation is obtained by quadrature-phase feeding of specially shaped dipole-and-reflector assemblies in orthogonal vertical planes, each having a generally hemispherical amplitude pattern of linearly polarized radiation. The hemispherical pattern ofeaeh assembly 908 is obtained by arcuate downwardly bent dipole ele- [56] References Cited ments each of physical length greater than a quarter wavelength and a horizontal reflector rod of length ex- UNITED STATES PATENTS ceeding a half wavelength, spaced downwardly of the 2,622,197 12/1952 Cruser 343/802 dipole feed-point by substantially more than a quarter 2,647,211 7/1953 Smeby 343/ 302 wavelength, but spaced downwardly of the tip portions 2516706 7/1950 Lapm't 343/798 of the dipole elements by substantially less than a quar- 3,64l,578 2/1972 Spanos 343/846 ter wavalength 3,426,351 2/1969 Hai et al. 343/797 3,579,244 5/1971 Dempsey et al. 343/797 4 Claims, 5 Drawing Figures REFLECTOR OMNIDIRECTIONAL ANTENNA This invention relates to omnidirectional antennas, and particularly to antennas having very large solid angle equal-radiation-intensity patterns, notably hemispherical'patterns.

Various antennas have heretofore been devised in attempts to approximate a hemispherical radiation pattern, particularly with circularly polarized radiation. Although such a pattern is of wider utility, the need for an antenna with such pattern characteristics has most notably existed in ground-to-air communications, and particularly in ground communications with satellites, in which it is often desirable to employ a single ground antenna, without necessity for tracking motions, for all upward azimuthal and elevational directions.

Circular polarization is normally desirable in satellite communication systems to make the communications link independent of the momentary orientation of the mobile antenna. Antennas most closely approaching satisfactory hemispherical radiation patterns with circular polarization have heretofore been of the type employing helical radiating elements. Various forms of helix, cylindrical and conical, have been employed, often in arrays, since most helical radiator configurations individually produce patterns which are reasonably uniform only over relatively narrow solid angles. So far as is known, the closest approach to the desired hemispherical radiation pattern which has been achieved prior to the present invention is obtained by the resonant quadrifilar helix described in IEEE Transactions on Antennas and Propagation (Communications), May, 1969, page 349.

It is of course well known that a circular polarization is generated from linearly polarized radiations orthogonal in direction and excited in phase quadrature. A pair of ordinary dipoles with mutually perpendicular radiating elements excited 90 out of phase can be employed for generation of circular polarization. However, since the appearance of'a circular polarization is reliant upon equality and perpendicularity of the fields from the two out-of-phase dipoles, the radiation pattern of useable polarized radiation is limited to' the narrow common axial region. .Attempt has heretofore been made to produce circularly polarized radiations of reasonably constantintensity over large upward solid. angles by modifying the configuration of quadrature-driven orthogonal dipoles. In the most successful of these previously known, the quarter-wave dipole elements are tilted upward so that each dipole forms a V. Reflector rods are employed at a distance of approximately a half wave downward from the feed-point to reduce the axial intensity and thus aid in horizontal diffusion as well as blocking downward radiation. The performance ob tained, however, falls short of closely approaching the desired uniformity of polarized radiation from horizon to horizon in all azimuthal directions.

The construction of the present invention is similar to that just described, in that it employs radiating elements in orthogonal planes, each of which constitutes a modified form of dipole. (It will of course be understood that the latter term as herein used refers to a single dipole, not to the end-connected parallel dipoles of the folded dipole antenna.) The radiating elements, however, rather than being linear, have portions or elementary lengths parallel to the dipole axis as well as portions or length elements perpendicular thereto.

As hereinafter more fully discussed, each dipole-andreflector assembly of such a construction produces a pattern of linearly polarized radiation. However, unlike the case of the linear half-wave dipole, the direction of polarization at remote points in the radiation field is a function of azimuthal and elevational direction. In the case of the ordinary linear dipole, the direction of polarization is parallel with the dipole at all points in space. With a dipole of the present construction, in which each of the radiating elements has at least one outer portion of its length extending parallel with the dipole axis, the far-field direction of polarization has no such uniformity. In the plane of the dipole, the direction of polarization varies from horizontal at the zenith to vertical at the horizon, with a continuum of directions at intermediate elevation points. In the plane orthogonal to the dipole plane, the electric-vector direction is horizontal at all points. When there is added a second such dipole in the orthogonal plane, the polarizations at the zenith remain the same as those produced by two conventional dipoles. At the horizon, in either of the dipole planes, the radiation is vertically polarized from one dipole and horizontally polarized from the other. When the two dipoles are driven in quadrature, their combined effect is a circular polarization where their fields are equal, the individual directions of polarization each varying with direction from the antenna, but remaining mutually perpendicular.

As a further aspect of the invention, based on empirical trial of various forms of the dipole construction just described, the invention provides a novel shaping and dimensioning for a dipole as so described, and its associated reflector, which produce substantially constant intensity of radiation over an entire hemisphere, so that a hemispherical pattern of circularly polarized radiation is produced by the combination of two dipoles. In addition to an inner end portion extending laterally outwardly from the axis or feed-point, and a portion outward of said inner end portion which extends downwardly, each radiating element has an outer end or tip portion extending inwardly and downwardly and a reflector element is disposed downward thereof. Each radiating element is of a length between a quarterwavelength and a half-wavelength and the outer end portions terminate slightly outward of the center or axis and less than a quarter-wave downwardly of the feedpoint, the reflector being less than a quarterwavelength downward of the termination of the radiating elements but more than a quarter-wavelength downward of the feed-point. The radiating elements are bent to the form of a continuous curve, most desirably a circular arc;

Further and more detailed features of construction constituting the teachings of the invention, and the mode of operation andadvantages thereof, will be best understood from the description below in connection with the illustrations of the drawing, in which:

FIG. 1 is a schematic view of a dipole-and-reflector assembly producing an approximately hemispherical pattern of linearly polarized radiation in accordance with the invention,

FIG. 2 is a fragmentary top plane view (partially in section taken along line 2-2 of FIG. 3) of a hemispherical-pattem antenna employing two sets of the basic elements illustrated schematically in FIG. 1

FIG. 3 is a front view in elevation of the antenna of FIG. 2;

FIG. 4 is a fragmentary sectional view taken along the line 4-4 of FIG. 3; and

FIG. 5 is a polar chart illustrating the vertical-plane radiation pattern of the antenna of FIGS. 2 through 4.

In the schematic diagram of FIG. 1 there is shown a dipole-and-reflector assembly consisting of balanced radiating elements and 12, center fed at 14 and pro vided with a reflector rod 16. Each of the radiating elements is bent to an are, forming slightly less than a semicircle, and is of length somewhat exceeding a quarter wavelength, but less than a half wavelength, the diameter of the approximate loop thus formed being somewhat less than a quarter wavelength. Each radiator element accordingly has an inner portion extending generally laterally outwardly from the feed-point, a portion outward thereof extending generally downwardly, and a tip portion extending generally inwardly and downwardly. The reflector rod, of length greater than a half-wavelength, is less than a quarter wavelength from the tip portions of the radiators and is between a quarter-wavelength and a half-wavelength below the feed-point.

The three-dimensional radiation pattern of the illustrated dipole elements may be roughly described from the standpoint both of intensity and polarization direction by comparison with the well-known pattern of a conventional half-wave dipole. In the plane of the loop, the E-plane, the cosine pattern of a linear dipole is drastically altered by the downward bending of the radiating elements. The nulls which appear in the directions of the ends of a linear dipole in this plane are eliminated and the intensity in the direct upwarddirection is greatly reduced. The reflector rod 16, which is longer than a half-wavelength, is spaced less than a quarter wavelength downward of the tips of the radiating elements but considerably more than a quarter wavelength (but less than a half wavelength) downward of the dipole feed-point, and even more broadly distributes the upward radiation pattern (as contrasted with the opposite function of a conventional reflector) in addition to blocking downward radiation.

In the plane perpendicular to the loop, the H-plane,

the circularly symmetrical pattern of a reflectorless linear dipole is likewise drastically altered by the phase relations of the currents contributing to the pattern in this plane. The reflector, which is of a length greater than twice the maximum extension of the radiating elements from their center, of course largely prevents radiation in rearward directions.

The direction of polarization of the radiation at any point in space is of course not constant. At any point in the E-plane, the polarization direction is in this plane, and is horizontal at the zenith and vertical at the horizon. In the H-plane, the polarization at any point is horizontal, since the vertical components of radiationcurrents are in opposite phase and self-cancelling. In intermediate azimuthal planes, the direction of polarization is neither in, nor perpendicular to, the plane but an intermediate angle which varies with both azimuth and elevation. At the horizon, there is azimuthal progression of the polarization direction from the vertical polarization in the E-plane to the horizontal polarization in the H-plane. In the plane at any given intermediate azimuthal angle, the direction of polarization changes with elevation from the tilted angle at the horizon to the E-plane orientation at the zenith.

Calculation of the exact polarization direction in planes intermediate between the E-plane and the H- plane is even more difficult than calculation of the intensity pattem. However, at least to a first approximation, at any elevation at which the intensities in the E- plane and the H-plane are equal (although of different polarization directions), the polarization direction angle varies linearly with azimuthal angle and the radiation intensity is essentially constant. By proper shaping and dimensioning of the dipole elements and reflector, this condition is substantially met throughout the hemisphere.

Shaping and dimensioning to obtain closely matched E-plane and I-I-plane patterns of such a dipole-andreflector' assembly may be accomplished experimentally, the difficulty of such experimentation varying with the performance specification sought to be met. For large equal-intensity solid angles, shaping of the radiating elements as circular arcs is desirable, particularly where a linear reflector is employed. The length of each arm or radiating element should be between a quarter wavelength and a half wavelength, and is most desirably between 0.30 and 0.40 wavelength. The radius of the circular arc should be from 0.05 to 0.2 wavelength, and the are described by each element should be from to 178, and preferably from to 178. The reflector is usually from 0.55 to 0.75 wavelength.

When a second assembly such as shown in FIG. 1 is added in the orthogonal plane, its pattern is of course the same both as regards intensity or amplitude and as regards polarization direction, except for the 90 displacement in horizontal planes. Thus at all elevations wherein the polarization direction angle changes linearly with azimuthal angle, the polarization directions from the two dipoles are always mutually perpendicular. Accordingly, when two such assemblies are fed in phase quadrature, there results a circular polarization within the entire equal-amplitude solid angle.

In FIGS. 2 through 4 there is shown the physical construction of an antenna incorporating the above principle which has further structural features provided by the invention. The assembly includes the radiating elements 10 and 12 and the reflector rod '16, with substantially identical orthogonal radiating elements 18 and 20 and areflector 22. The assembly is mounted on a sup port post 24 having at its top a base-plate 26 from which there extends upwardly a support and balun assembly generally designated at 28. This assembly is formed by four vertically extending angle-bars 30a, 30b, 30c and 30d welded to the base-plate 26. The bars are braced in properly spaced relation by dielectric spacers32 and 34 spacing the opposed faces of adjacent bars. The inner ends of the radiating elements are chamfered to fit into the angles of the respective bars and welded therein. The assembly is fed by an input coaxial cable 36 terminating in a T-connection or power divider 38 which feeds separate coaxial cables 40 and 42, the former being sufficiently longer than the latter to produce the desired quadrature relation of the feeds of the respective dipoles. The outer conductors of the cables 40 and 42 are connected to respective bars 300 and 30d by suitable grounding clips 44. The inner portion of the end of cable 42 extends through suitable apertures in the angle bars 30b and 30d enclosed in an insulating sleeve 46 and the inner portion 48 of the cable 40 similarly extends through the diagonally opposed bars 30a and 30c, each of the inner conductors being connected to the bar thereby fed by a suitable terminal connection 50. The angle bars serve as baluns, as well as supports, for the respective pairs of dipole elements, matching the impedance of the coaxial lines to that of the dipoles.

The construction shown in the drawing and described above has been employed in an antenna for operation at 249MHz with the following construction details: Each of the radiating elements was formed by aluminum pipe of 1.05 inch outer diameter, of a length of approximately 16 inches (approximately one-third of a wavelength) bent to an arc of a circle of diameter slightly less than inches (approximately one-fifth of a wavelength). Each reflector was of a length of 31 inches (approximately two-thirds of a wavelength) and spaced downwardly from the tip portions of the dipole by approximately 5 inches (roughly one-tenth of a wavelength). The distance from the top or feed points of the dipoles to the reflector was inches (a little less than one-third wavelength).

The measured intensity pattern of the antenna just described in the planes of the two dipoles was as shown in FIG. 5, from which it may be seen that the maximum variation in intensity over the entire 180 range from horizon to horizon was approximately 2.5 dB. The halfpower point of the generally cardioid pattern is below the horizon. Curves or patterns for intermediate planes were only slightly less ideal. Circularity of polarization was also measured in terms of the axial ratio (or ellipticity of the circular polarization, i.e., the ratio of maximum to minimum with linear dipole rotation). The axial ratio was in no case poorer than 3 dB in the hemisphere and was considerably less than 2 dB at all points substantially upward of the horizon.

Persons skilled in the art may readily devise variants which utilize the basic teachings of the invention. For a hemispherical pattern, as already indicated, circular arcs should be employed of curvature radius from 0.05 to 0.2 wavelength, each arc being between 140 and 178, and the length of the reflector being from 0.55 to 0.75 wavelength. However for less exacting applications, or for special-shaped patterns, the principle may be employed in other forms.

For convenience of reference, the foregoing descrip-- tion of the invention, and the appended claims, refer to a particular orientation of the antenna wherein the antenna axis is vertical, by reason of the employment for satellite communications for which the invention was devised, and, correspondingly, the axial directions are referred to as upward and downward and the lateral direction as the horizon. It will be recognized by those skilled in the art, however, that the invention has utilities wherein the antenna is differently oriented, so that it will be understood that terms such as upward," downward, etc., should not be considered limitative as to antenna orientation. Nor shall the protection afforded the invention be otherwise limited to the particular embodiment herein described. The scope of the invention should accordingly be determined in accordance with the definitions thereof in the appended claims, and equivalents thereto.

What is claimed is:

1. An antenna for producing a large solid-angle radiation pattern comprising:

a. a balanced center-fed dipole radiator assembly having a pair of radiating elements each having an inner end portion extending generally laterally outwardly from the feed-point, a portion outward of said inner end portion extending generallydownwardly and a tip portion extending generally inwardly and downwardly and having no conductive connection between the respective tip portions, and

b. a reflector element at least partially in the plane of the radiator elements and downward thereof and of a dimension in said plane greater than twice the maximum extension of the radiating elements from their center, said reflector being a transversely extending rod of a length greater than a halfwavelength.

2. The antenna of claim 1 characterized by each of said radiating elements being of a length between 0.30 and 0.40 wavelength and being bent to the form of a continuous curve.

3. The antenna of claim 2 characterized by each of said radiating elements being of a length between 0.30 and 0.40 wavelength and being bent to the form of a circular arc substantially greater than 4. The antenna of claim 3 characterized by the circular arc of each element being between and 178 of a circle of radius from 0.05 to 0.2 wavelength, and the length of the reflector being from 0.55 to 0.75

wavelength. 

1. An antenna for producing a large solid-angle radiation pattern comprising: a. a balanced center-fed dipole radiator assembly having a pair of radiating elements each having an inner end portion extending generally laterally outwardly from the feed-point, a portion outward of said inner end portion extending generally downwardly and a tip portion extending generally inwardly and downwardly and having no conductive connection between the respective tip portions, and b. a reflector element at least partially in the plane of the radiator elements and downward thereof and of a dimension in said plane greater than twice the maximum extension of the radiating elements from their center, said reflector being a transversely extending rod of a length greater than a halfwavelength.
 2. The antenna of claim 1 characterized by each of said radiating elements being of a length between 0.30 and 0.40 wavelength and being bent to the form of a continuous curve.
 3. The antenna of claim 2 characterized by eAch of said radiating elements being of a length between 0.30 and 0.40 wavelength and being bent to the form of a circular arc substantially greater than 90*.
 4. The antenna of claim 3 characterized by the circular arc of each element being between 140* and 178* of a circle of radius from 0.05 to 0.2 wavelength, and the length of the reflector being from 0.55 to 0.75 wavelength. 